1. Field of the Invention
The present invention relates to a power tool that performs a predetermined operation by driving a tool bit.
2. Description of the Related Art
Japanese laid-open patent publication No. 2009-101500 discloses a screw tightening machine having a multi-plate friction clutch mechanism in which a plurality of friction plates are layered in its longitudinal direction between a driving part which is driven by a driving motor and a driven part to which a tool bit is attached. According to the screw tightening machine having the above-described construction, when the tool bit in the form of a bit is pressed against a head of a screw, a plurality of clutch plates come in contact with each other by reaction force caused by this pressing operation and frictional force is caused. As a result, torque of the driving part is transmitted to the bit via the multi-plate friction clutch mechanism and a screw tightening operation is performed by the bit.
Because the multi-plate friction clutch mechanism disclosed in the above-described publication requires a certain number of clutch plates in order to transmit a certain torque, a number of clutch plates are layered in the longitudinal direction. As a result, the length of a tool body tends to increase in the longitudinal direction, and when the above-described pressing operation is released, the clutch plates tend to be kept in contact with each other and easily cause dragging. In this point, further improvement is desired.